refactor: UI polish — lock sidebar, remove nav buttons, uniform artist tables, deep shuffle

- Lock sidebar width (setFixedWidth) so it doesn't jump between views
- Remove back/forward navigation buttons and all NavPage history code
- Uniform column layout on artist page: hide Artist column from both
  Popular Tracks and release sections, set matching fixed column widths
  so columns align vertically across all sections
- Deep shuffle: new Rust FFI endpoint fetches tracks from all albums
  in parallel, combines them, and returns via EV_DEEP_SHUFFLE_OK
- Auto-paginate artist releases in Rust (loop until has_more=false)
  so all releases load at once sorted newest-first

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

rust/include/qobuz_backend.h

@@ -35,6 +35,8 @@ enum QobuzEvent {
     EV_PLAYLIST_DELETED      = 21,
     EV_PLAYLIST_TRACK_ADDED  = 22,
     EV_USER_OK               = 23,
+    EV_ARTIST_RELEASES_OK    = 24,
+    EV_DEEP_SHUFFLE_OK       = 25,
 };
 
 // Callback signature
@@ -84,9 +86,12 @@ uint32_t qobuz_backend_viz_read(QobuzBackendOpaque *backend, float *buf, uint32_
 uint32_t qobuz_backend_viz_sample_rate(const QobuzBackendOpaque *backend);
 uint32_t qobuz_backend_viz_channels(const QobuzBackendOpaque *backend);
 
-// Artist releases (full paginated list per release type)
+// Artist releases (auto-paginates to fetch all)
 void qobuz_backend_get_artist_releases(QobuzBackendOpaque *backend, int64_t artist_id, const char *release_type, uint32_t limit, uint32_t offset);
 
+// Deep shuffle: fetch tracks from multiple albums (album_ids_json is a JSON array of strings)
+void qobuz_backend_get_albums_tracks(QobuzBackendOpaque *backend, const char *album_ids_json);
+
 // Playlist management
 void qobuz_backend_create_playlist(QobuzBackendOpaque *backend, const char *name);
 void qobuz_backend_delete_playlist(QobuzBackendOpaque *backend, int64_t playlist_id);